Abrupt Climate Change
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The United Nations Framework Convention on Climate Change can be viewed at .
The National Research Council (NRC) report Abrupt Climate Change: Inevitable Surprises (65) provides a more comprehensive treatment of abrupt climate change with over 650 references. The members of the Panel on Abrupt Climate Change which prepared the NRC report are the authors of this review. The recommendations of the NRC report: Improve the fundamental knowledge base modeling instrumental and paleoclimatic data and statistical approaches related to abrupt climate change and investigate “no- regrets” strategies to reduce vulnerability. The report is available at .
Short-term climate stability is provided by the increase in longwave radiation emitted by Earth as it warms and reduction in emitted radiation as it cools. The large heat capacity and specific heats of water also contribute to very short term stability. Very long term stability likely occurs because the rate of production of CO 2 from volcanoes is nearly independent of Earth's surface temperature but the rate at which CO 2 is removed from the atmosphere by chemical reaction with rocks increases with temperature which increases with atmospheric CO 2 (66).
W. S. Broecker The Glacial World According to Wally (Eldigio Lamont-Doherty Earth Observatory of Columbia University Palisades NY ed. 3 2002).
Ice-age cycles were caused by orbitally induced latitudinal and seasonal redistribution of sunlight that led to changes in the amount of sunlight reflected by Earth (through changes in snow and ice vegetation and probably clouds and dust) in the greenhouse-gas concentration of the atmosphere (primarily CO 2 and water vapor but including CH 4 and N 2 O) and perhaps in other factors (4).
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C. T. Morrill J. T. Overpeck J. E. Cole Holocene in press.
E. J. Brook S. Harder J. Severinghaus M. Bender in Mechanisms of Global Climate Change at Millennial Time Scales P. U. Clark R. S. Webb L. D. Keigwin Eds. (Geophysical Monograph 112 American Geophysical Union Washington DC 1999) pp. 165–176.
North Atlantic records show a repeated pattern often with ∼1500-year spacing of abrupt warming followed by gradual cooling abrupt cooling and a few cold centuries. Generally cold dry and windy conditions occurred together across much of the Earth although with antiphase behavior in some far southern regions. The anomalously mild times following the abrupt warmings are often called Dansgaard/Oeschger (DO) events but here we follow some workers in referring to the DO oscillation without necessarily implying strict periodicity (6). At least some of the cold phases immediately followed floods or ice-sheet surges into the North Atlantic (4) including a centennial cold event about 8200 years ago with widespread impacts (45) that immediately followed a large outburst flood from a lake dammed by the melting ice sheet in Hudson Bay (67).
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This difficulty lack of a globalizer is shared with the standard explanation of global ice-age cooling by reduced Northern Hemisphere summer insolation from the relatively weak 100 000-year cyclicity of orbital forcing (4).
Except for the event about 8200 years ago the Holocene changes differ from the DO oscillations in many ways with Holocene changes smaller of less clear but probably reduced spatial extent and uniformity and lacking the global abrupt perturbations of biogeochemical cycles shown by shifts in trace gases such as CH 4 N 2 O and CO 2 in the ice-age events (6).
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J. T. Overpeck C. Whitlock B. Huntley in Paleoclimate Global Change and the Future K. Alverson R. Bradley T. Pedersen Eds. (IGBP Synthesis Volume Springer-Verlag Berlin 2003) pp. 81–111.
Ecosystems and economies can be forced across thresholds by gradual as well as by abrupt climate changes causing major abrupt impacts although faster forcing is probably more likely to cross impacts thresholds.
IPCC (Intergovernmental Panel on Climate Change) Climate Change 2001: Impacts Adaptation and Vulnerability. Report of Working Group II (Cambridge Univ. Press Cambridge UK 2001); available at www.ipcc.ch.
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IPCC (Intergovernmental Panel on Climate Change) Climate Change 2001: The Science of Climate Change. Report of Working Group I (Cambridge Univ. Press Cambridge UK 2001); available online at www.ipcc.ch.
One prominent warm interval was the Paleocene-Eocene Thermal Maximum (68) which began with warming over perhaps 10 000 to 20 000 years or faster of about 4° to 8°C in high-latitude ocean surface temperatures and 4° to 6°C in bottom-water temperatures from conditions that were already warmer and with an equator-to-pole temperature gradient that was smaller than occurred recently. A change in location of deep-water formation may have led to massive destabilization of methane hydrate in sea-floor sediments. Impacts included extinction of 30 to 50% of benthic foraminifera and subtropical drying.
Freshening may be arising from one or more processes including increased high-latitude precipitation or fraction of precipitation running off the land (69) melting of sea or land ice or changes in wind-driven or other exchange with the Arctic Ocean; the complexity is challenging for modern observations and models (16).
Seager et al. (70) emphasized that the relative warmth of the northeastern versus northwestern Atlantic arises only in part from the thermohaline circulation; thus any discussions of the possible effects of a thermohaline shutdown that cite the Norway-Canada difference may be overstated. Nonetheless the thermohaline circulation does transport much heat to and affect the climate of the North Atlantic (4 70). The tendency of many models to underestimate abrupt paleoclimatic changes leaves open the possibility that other discussions have underestimated the potential effects of a thermohaline shutdown. The need for improved research to address these issues is clear.
J. Marotzke in Decadal Climate Variability: Dynamics and Predictability D. L. T. Anderson J. Willebrand Eds. (Springer-Verlag Berlin 1996).
Abrupt Climate Change: Inevitable Surprises (National Research Council National Academy Press Washington DC 2002).
K. R. Laird S. C. Fritz K. A. Maasch B. F. Cumming Moon Lake Diatom Salinity-Drought Data (IGBP PAGES/World Data Center-A for Paleoclimatology Data Contribution Series #1998-015 National Oceanic and Atmospheric Administration–National Geophysical Data Center Paleoclimatology Program Boulder CO 1998).
We thank NRC staff (A. Isern J. Dandelski C. Elfring M. Gopnik M. Kelly J. Bachim A. Carlisle) the U.S. Global Change Research Program and the Yale National Bureau of Economic Research on International Environmental Economics for study funding sponsors of our research (including NSF OPP 0087160 to R.B.A.) the community of researchers studying abrupt climate change who made this possible and especially D. Bradford W. Curry and K. Keller for helpful comments.